The purpose of this study is to evaluate the tumor response of stable disease (SD), partial response (PR), or complete response (CR) [according to Response Evaluation Criteria In Solid Tumors version 1.1 (RECIST v1.1 criteria)] at 12 weeks in participants with Gastrointestinal Stromal Tumors (GIST) harboring platelet-derived growth factor receptor alpha (PDGFRα) mutations and patients with GIST not harboring PDGFRα mutations.